Engines may be operated with boosted aircharge provided via a turbocharger wherein an intake compressor is driven by an exhaust turbine. However, placing a turbine in an exhaust system can increase engine cold-start emissions due to the turbine acting as a heat sink. In particular, engine exhaust heat during the engine cold-start may be absorbed at the turbine, lowering the amount of exhaust heat that is received at a downstream exhaust catalyst. As such, this delays catalyst light-off. Consequently, spark retard may be required in order to activate the exhaust catalyst. However, the fuel penalty associated with the spark retard usage may offset or even outweigh the fuel economy benefit of the boosted engine operation.
Accordingly, various approaches have been developed to expedite the attainment of a catalyst light-off temperature during cold-start conditions in a boosted engine. One example approach, shown by Andrews in U.S. Pat. No. 8,234,865 involves routing exhaust gas towards an exhaust tailpipe via a passage that bypasses the exhaust turbine during cold-start conditions. A passive, thermatically operated valve is used to regulate the flow of exhaust through the passage, the valve opening during low-temperature conditions (such as during cold-start). The thermatically operated valve comprises a bi-metallic element which distorts based on temperature thereby regulating the opening of the valve. By circumventing the turbine, exhaust heat may be directly delivered to the exhaust catalyst.
However, the inventors herein have recognized potential issues with such systems. As one example, due to the exhaust bypassing the turbine, there may be a delay in turbine spin-up, resulting in turbo-lag and reduced boost performance. Furthermore, after catalyst light-off, the temperature of the unobstructed exhaust reaching the catalyst may be higher than desired. In particular, owing to a coating on the catalyst surface (such as on the surface of an exhaust oxidation catalyst or three-way catalyst), the catalyst may have higher conversion efficiencies at lower exhaust temperatures. As a result, the higher than desired temperature of exhaust reaching the catalyst may result in reduced catalyst functionality.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method for a boosted engine comprises: during an engine cold-start, flowing exhaust first through a three-way catalyst then through an underbody converter and then a turbine; after catalyst light-off, flowing exhaust first through the turbine, then through the underbody converter, and then through the three-way catalyst; and during high load operation, bypassing the turbine at least partially. In this way, exhaust heat may be used to reduce turbo lag while expediting catalyst light-off.
In one example, a turbocharged engine system may be configured with a branched exhaust assembly wherein the exhaust passage is divided into at least three separate branches, each creating a distinct flow path. The branches may be interconnected to each other via valves such that an order of exhaust flow along each of the flow paths can be adjusted via adjustments to a position of the valves. Distinct exhaust components may be coupled to distinct branches of the branched exhaust assembly. For example, an exhaust turbine of the turbocharger may be coupled to a first branch, an underbody convertor may be coupled to a second branch, and an exhaust oxidation catalyst (three-way catalyst) may be coupled to a third branch of the exhaust assembly. During cold-start conditions, the valves may be adjusted to flow exhaust through the catalyst, then through the underbody converter, and then through the turbine. After catalyst light-off, the valves may be adjusted to flow exhaust first through the turbine, then through the underbody converter, and then through the catalyst. During high engine load conditions, such as while operating with boost, the valves may be adjusted such that exhaust may be simultaneously routed to the tailpipe through two separate flow paths. For example, a first portion of exhaust gas may flow first through turbine, then through the underbody converter, and then through the light-off catalyst before exiting via the tail pipe. A second (remaining) portion of the exhaust gas may directly flow through the light-off (activated) catalyst bypassing the turbine and the underbody converter before exiting via the tail pipe. The portion of the exhaust routed through the catalyst relative to the portion routed through the turbine is adjusted based on engine load.
In this way, by routing exhaust through different flow-paths of a branched exhaust assembly, it is possible to expedite attainment of catalyst light-off temperature while providing boost to the engine during cold-start conditions. Specifically, exhaust can be flowed through each of a turbine, an exhaust catalyst, and an underbody converter, with an order of exhaust flow through the components adjusted based on operating conditions. By adjusting exhaust flow during cold-start conditions to route hot exhaust through an exhaust catalyst, before flowing the exhaust through the remaining exhaust components, exhaust heat may be effectively transferred to the catalyst, expediting catalyst activation. By adjusting the exhaust flow after catalyst activation to route the hot exhaust through an exhaust turbine before flowing the exhaust through the remaining exhaust components, turbo lag is reduced. In addition, a temperature of the exhaust received at the catalyst is lowered, improving catalyst conversion efficiency. By routing exhaust via multiple flow paths in the exhaust assembly, it is possible to partially bypass the turbine thereby reducing the possibility of boost error during high engine load conditions. The technical effect of using valves to regulate an order of exhaust flow through the exhaust components housed in distinct branches of the branched exhaust assembly is that exhaust heat can be directed to a specific component first, as required based on engine operating conditions, irrespective of the order of the exhaust components relative to each other in the exhaust assembly. Overall, by changing an order of exhaust flow through exhaust components, engine efficiency, emissions quality, and fuel efficiency may be improved in a boosted engine system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.